Blender Motion Blur deactivation for certain objects doesn´t work
Vector Blur Solution
When using the Vector pass and the Vector Blur node, individual objects can easily be &quot;unblurred&quot; by mixing in the raw render which has no blur.
To enable the Vector pass, Motion Blur must be disabled.

The Vector pass an be enabled in Properties &gt; View Layer Properties &gt; Passes &gt; Vector.

I have also enabled Object Index for creating the object mask, but using Cryptomatte would work even better.
My boat proxy object has its Pass Index set to 1 for this mask.

In the compositor, the object's mask can now be used to zero the Vector pass and mix in the raw render where necessary as seen in the node setup below.

The Dilate/Erode node is used to reduce the color bleed artifacts from the Vector Blur node.
Side-By-Side

my proxy scene with the blur removed on the right side of the arrow object